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.NET Forum / Languages / C# / August 2006Generics constraint for nullable or reference types
Hello, I want to define a generic class which should accept only nullable types or reference types. What's the best way to costrain it? --------- class MyClass<T>{ ... } --------- T must be either nullable type or reference type. int? is ok. string is ok. int is not ok. Thanks in advance. Sam Well, you can't do it easily - there is no single constraint that would help here. Is there any way you can split the code out into 2 blocks, one for class, one for Nullable<T>? You would still need to use Default<T> for the "null", since the (class) null-reference and Nullable<T> "null" are IIRC quite different with the compiler making them look the same(Nullable<T> being a struct). Marc You can constrain what types your class can be instantiated with by using the where clause as in: class MyClass<T> where T : class I cannot see an obvious way to constrain the type to being a Nullable type as int?, Nullable<int> etc. all appear to be invalid in the where clause above! - Andy > Hello, > [quoted text clipped - 19 lines] > > Sam I've done some more digging and the problem is that Nullable is actually a struct. Accordingly templates can only be derived from interfaces, non-sealed CLASSES and types. struct's cannot be used as base classes; full stop... You may be able to define your own class which implements the methods in line with how the Nullable struct operates; it basically adds a boolean to each type to indicate whether the value has been assigned or not... I cannot see any reason why this will not work but you may not be able to use the short hand form; i.e. type? you may have to use MyType.Nullable<int> myInt... as I would assume that the short hand form would be bound to System.Nullable (could be wrong :). I cannot think of any reason why this will not work but unless I actually tried to write the code could not guarantee it would be trouble free. I also cannot think of any reason why Nullable is implemented as a struct aside from a value type performance angle. If you wrote your own Nullable class then: class MyClass<T> where T : class Would obviously work with both reference types and your new MyNameSpace.Nullable class (as it would be a class) the only problem would be that calling the methods on it would need an is check (i.e. if (t is MyNameSpace.Nullable)...). To avoid this you would need to define two classes, one for reference types and the other for your Nullable type: class MyRefClass<T> where T : class class MyNullableClass<T> where T : MyNameSpace.Nullable HTH - Andy > You can constrain what types your class can be instantiated with by using > the where clause as in: [quoted text clipped - 30 lines] >> >> Sam > I've done some more digging and the problem is that Nullable is actually a > struct. Accordingly templates can only be derived from interfaces, [quoted text clipped - 33 lines] > > - Andy Thank you for your kind answer. Actually I was trying to make a generic class like this. class Board<T>{ T[,] cells; ... } Board instances holds some 2D array of a type. You may think of it as Sudoku board. But I wanted to make it more generic so that strings or numbers can be contained (not the same time though). So it will be like string[,] or int[,]. But for empty cells, I want to use null instead of 0.(0 doesn't always mean empty) So it will be string[,] or int?[,]. But I couldn't find a way to define that way. If I don't define such constraint, I can't set null to a cell like cells[0, 0] = null;. The compiler would complain that cells[0,0] might be a value type. For now, I just made it for int? only. Thanks. Sam Hi - Okay; I'm not sure that you need to place a constraint on the generic as the compiler will do all the work for you. Consider: class MyBoard<T> { T[,] t; public MyBoard(int r, int c) { this.t = new T[r, c]; } public T this[int x, int y] { get { return this.t[x, y]; } set { this.t[x, y] = value; } } } I can use this class with any type (either value or reference) and as long as I'm careful to copy reference values can do things such as: MyBoard<int> mb = new MyBoard<int>(2, 2); mb[0, 0] = 2; mb[1, 1] = null; // *Compiler error* int a = mb[0, 0]; // Should be 2. MyBoard<int?> mb = new MyBoard<int?>(2, 2); mb[0, 0] = 2; mb[1, 1] = null; int? a = mb[1, 1]; // Should be null. MyBoard<string> mb = new MyBoard<string>(2, 2); ... The indexer is the key to getting/setting items into the array as this only works with the type the class is expecting. - Andy >> I've done some more digging and the problem is that Nullable is actually >> a [quoted text clipped - 69 lines] > > Sam Hi Andy, > Hi - > [quoted text clipped - 35 lines] > The indexer is the key to getting/setting items into the array as this only > works with the type the class is expecting. Yes, that works. However, I can't assign null to the array in MyBoard class. Let's say, you want to reset the board, you might do public void Reset(){ for(int x = 0; x < t.GetLength(0); x++) for(int y = 0; y < t.GetLength(1); y++) t[x, y] = null; //Compilation Error!!! } Well, I can reset it outside the board class. But that's not a very good design, IMHO. Any idea? Thanks. Sam Easy to correct the compilation error, change the line: t[x, y] = null; //Compilation Error!!! to t[x, y] = default(T); This will create an appropriate empty value. For reference types and nullable types it's null, for value types it's 0 and for structs it's a copy of the struct with each field to to 0. Information on this can be found here: http://msdn2.microsoft.com/en-us/library/xwth0h0d.aspx That keeps the class nicely encapsulated and resolves the problem that you are getting instantiating your generic class with value types and structs. We got there in the end! - Andy > Hi Andy, > [quoted text clipped - 58 lines] > > Sam > Easy to correct the compilation error, change the line: > [quoted text clipped - 3 lines] > > t[x, y] = default(T); Thank you very much Andy. Now I will modify my Board class with that. You're really helping. Thanks again. Sam Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. 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